Falsely accusing an opponent or
potential critic of the exact thing that you are saying or doing before they get a chance to call you out on
what you are saying or doing.
Thus, bystanders and third parties become confused as to what is real and what isn't, and you stand a chance of getting away with
what you are saying or doing.
He told a lie and then
immediately said "but the fake news media will lie and disagree with me". Now
the press can't call him on his lie without looking like liars themselves and neutral bystanders feel like they have to take a middle-ground. That's some shady liejacking
right there.